Method and Apparatus of Electrical Property Measurement Using an AFM Operating in Peak Force Tapping Mode

ABSTRACT

An apparatus and method of collecting topography, mechanical property data and electrical property data with an atomic force microscope (AFM) in either a single pass or a dual pass operation. PFT mode is preferably employed thus allowing the use of a wide range of probes, one benefit of which is to enhance the sensitivity of electrical property measurement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 13/925,385, filed on Jun. 24, 2013 (U.S. Pat. No.9,213,047, issued Dec. 15, 2015), which claims priority to ProvisionalPatent Application No. 61/663,528, filed on Jun. 22, 2012, both of whichare entitled Method and Apparatus of Electrical Property MeasurementUsing an AFM Operating in Peak Force Tapping Mode. U.S. Non-Provisionalpatent application Ser. No. 13/925,385 is also a continuation-in-part ofand claims priority to U.S. Non-Provisional patent application Ser. No.13/306,867, filed on Nov. 29, 2011 (U.S. Pat. No. 8,650,660, issued Feb.11, 2014), which in turn, claims priority under 35 USC §1.119(e) to U.S.Provisional Patent Application No. 61/417,837, filed on Nov. 29, 2010,both entitled Method and Apparatus of Using Peak Force Tapping Mode toMeasure Physical Properties of a Sample. U.S. Non-Provisional patentapplication Ser. No. 13/925,385 is also a continuation-in-part of andclaims priority to U.S. patent application Ser. No. 12/618,641, filed onNov. 13, 2009 (U.S. Pat. No. 8,739,309, issued May 27, 2014), entitledMethod and Apparatus of Operating a Scanning Probe Microscope, which inturn, claims priority under 35 USC §1.119(e) to U.S. Provisional PatentApplication No. 61/114,399, filed Nov. 13, 2008. The subject matter ofthese applications is hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The preferred embodiments are directed to scanning probe microscopymethods and apparatus, and more particularly, using an atomic forcemicroscope (AFM) to collect topography, mechanical and electrical sampleproperty data, preferably using peak force tapping mode (PFT mode) AFMand Kelvin Probe Force Microscopy (KPFM), respectively.

2. Description of Related Art

Scanning probe microscopes (SPMs), such as the atomic force microscope(AFM), are devices which typically employ a probe having a tip and whichcause the tip to interact with the surface of a sample with low forcesto characterize the surface down to atomic dimensions. Generally, theprobe is introduced to a surface of a sample to detect changes in thecharacteristics of a sample. By providing relative scanning movementbetween the tip and the sample, surface characteristic data can beacquired over a particular region of the sample, and a corresponding mapof the sample can be generated.

A typical AFM system is shown schematically in FIG. 11. An AFM 10employs a probe device 12 including a probe 17 having a cantilever 15. Ascanner 24 generates relative motion between the probe 17 and a sample22 while the probe-sample interaction is measured. In this way, imagesor other measurements of the sample can be obtained. Scanner 24 istypically comprised of one or more actuators that usually generatemotion in three mutually orthogonal directions (XYZ). Often, scanner 24is a single integrated unit that includes one or more actuators to moveeither the sample or the probe in all three axes, for example, apiezoelectric tube actuator. Alternatively, the scanner may be aconceptual or physical combination of multiple separate actuators. SomeAFMs separate the scanner into multiple components, for example an XYactuator that moves the sample and a separate Z-actuator that moves theprobe. The instrument is thus capable of creating relative motionbetween the probe and the sample while measuring the topography or someother property of the sample as described, e.g., in Hansma et al. U.S.Pat. No. RE 34,489; Elings et al. U.S. Pat. No. 5,266,801; and Elings etal. U.S. Pat. No. 5,412,980.

In a common configuration, probe 17 is often coupled to an oscillatingactuator or drive 16 that is used to drive probe 17 to oscillate at ornear a resonant frequency of cantilever 15. Alternative arrangementsmeasure the deflection, torsion, or other characteristic of cantilever15. Probe 17 is often a microfabricated cantilever with an integratedtip 17.

Commonly, an electronic signal is applied from an AC signal source 18under control of an SPM controller 20 to cause actuator 16 (oralternatively scanner 24) to drive the probe 17 to oscillate. Theprobe-sample interaction is typically controlled via feedback bycontroller 20. Notably, the actuator 16 may be coupled to the scanner 24and probe 17 but may be formed integrally with the cantilever 15 ofprobe 17 as part of a self-actuated cantilever/probe.

As a selected probe 17 is oscillated, it is brought into contact withsample 22 as sample characteristics are monitored by detecting changesin one or more characteristics of the oscillation of probe 17, asdescribed above. In this regard, a deflection detection apparatus 25 istypically employed to direct a beam towards the backside of probe 17,the beam then being reflected towards a detector 26, such as a fourquadrant photodetector. The deflection detector is often an opticallever system such as described in Hansma et al. U.S. Pat. No. RE 34,489,but may be some other deflection detector such as strain gauges,capacitance sensors, etc. The sensing light source of apparatus 25 istypically a laser, often a visible or infrared laser diode. As the beamtranslates across detector 26, appropriate signals are processed by asignal processing block 28 (e.g., to determine the RMS deflection ofprobe 17). The interaction signal (e.g., deflection) is then transmittedto controller 20, which processes the signals to determine changes inthe oscillation of probe 17. In general, controller 20 determines anerror at Block 30, then generates control signals (e.g., using a PI gaincontrol Block 32) to maintain a relatively constant interaction betweenthe tip and sample (or deflection of the lever 15), typically tomaintain a setpoint characteristic of the oscillation of probe 17. Thecontrol signals are typically amplified by a high voltage amplifier 34prior to, for example, driving scanner 24. For example, controller 20 isoften used to maintain the oscillation amplitude at a setpoint value,A_(S), to insure a generally constant force between the tip and sample.Alternatively, a setpoint phase or frequency may be used. Controller 20is also referred to generally as feedback where the control effort is tomaintain a constant target value defined by the setpoint.

A workstation 40 is also provided, in the controller 20 and/or in aseparate controller or system of connected or stand-alone controllers,that receives the collected data from the controller 20 and manipulatesthe data obtained during scanning to perform data manipulation operatingsuch as point selection, curve fitting, and distance determiningoperations. The workstation can store the resulting information inmemory, use it for additional calculations, and/or display it on asuitable monitor.

AFMs may be designed to operate in a variety of modes, including contactmode and oscillating mode. Operation is accomplished by moving thesample and/or the probe assembly up and down relatively perpendicular tothe surface of the sample in response to a deflection of the cantileverof the probe assembly as it is scanned across the surface. Scanningtypically occurs in an “x-y” plane that is at least generally parallelto the surface of the sample, and the vertical movement occurs in the“z” direction that is perpendicular to the x-y plane. Note that manysamples have roughness, curvature and tilt that deviate from a flatplane, hence the use of the term “generally parallel.” In this way, thedata associated with this vertical motion can be stored and then used toconstruct an image of the sample surface corresponding to the samplecharacteristic being measured, e.g., surface topography. In onepractical mode of AFM operation, known as TappingMode™ AFM (TappingMode™is a trademark of the present assignee), the tip is oscillated at ornear a resonant frequency of the associated cantilever of the probe, orharmonic thereof. A feedback loop attempts to keep the amplitude of thisoscillation constant to minimize the “tracking force,” i.e., the forceresulting from tip/sample interaction, typically by controllingtip-sample separation. Alternative feedback arrangements keep the phaseor oscillation frequency constant. As in contact mode, these feedbacksignals are then collected, stored and used as data to characterize thesample.

Regardless of their mode of operation, AFMs can obtain resolution downto the atomic level on a wide variety of insulating or conductivesurfaces in air, liquid or vacuum by using piezoelectric scanners,optical lever deflection detectors, and very small cantileversfabricated using photolithographic techniques. Because of theirresolution and versatility, AFMs are important measurement devices inmany diverse fields ranging from semiconductor manufacturing tobiological research. Note that “SPM” and the acronyms for the specifictypes of SPMs, may be used herein to refer to either the microscopeapparatus or the associated technique, e.g., “atomic force microscopy.”

Kelvin-Probe Force Microscopy (KPFM), also known as Surface PotentialMicroscopy (SPoM), Surface Electric Potential Microscopy (SEPM), hasbeen an important tool for electrical measurements using scanning probemicroscopes (SPMs), such as AFMs, for many years.

Fundamentally, KPFM is a combination of atomic force microscopy (AFM)and Kelvin probe technique. Kelvin probe technique was designed tomeasure the contact potential difference (CPD) between an AFM probe anda sample surface when the two are brought close to one another. The CPDdepends largely on the work function difference between the twomaterials. In this regard, the work function of a sample under test canbe deduced if the work function of the probe is calibrated against asample having a well-defined work function. Traditional Kelvin probetechnique has a high sensitivity for potential measurements but offerspoor spatial resolution. The invention of atomic force microscope (AFM)by Binnig et al. in 1986 (U.S. Pat. No. 4,724,318) opened the door toimaging solid sample surfaces of all kinds with nanometer to atomicresolution. Weaver et al. adapted Kelvin probe technique and combined itwith AFM in 1991 (“High Resolution Atomic Force MicroscopyPotentiometry”, Weaver et al., J. Vac. Sci. Technol. B Vol. 9, No. 3,May/June 1991, pp. 1559-1561); Nonnenmacher et al. coined the termKelvin probe force microscopy shortly after (“Kelvin Probe ForceMicroscopy”, Nonnenmacher et al., Appl. Phys. Lett. Vol. 58, No. 25,June 1991, pp. 2921-2923). Thereafter, different AFM modes and KPFMdetection schemes have appeared, and the various combinations of themhave flourished the art.

Instead of measuring current as in Kelvin probe technique, KPFM is basedon force measurement, employing the sensitive force detection capabilityin an AFM. The AFM probe and sample together is modeled as a parallelplate capacitor, the force between is thus:

$F_{el} = {{- \frac{1}{2}}\frac{\partial C}{\partial z}\left( {\Delta \; V} \right)^{2}}$

where F_(el) is the electric force, C the capacitance, and ΔV thevoltage difference. ΔV can be the sum of CPD

$\left( \frac{\Delta\varphi}{e} \right),$

and the externally applied DC V_(DC) and AC voltage V_(AC) withfrequency f:

${\Delta \; V} = {V_{DC} - \frac{\Delta\varphi}{e} + {V_{AC}{\sin \left( {2{\pi {ft}}} \right)}}}$

Various methods for implementing KPFM have been proposed, a discussionof some of which follows.

(i) The so-called amplitude-modulation method (AM-KPFM) in which an ACvoltage between the probe and the sample excites a mechanicaloscillation of the cantilever. AM-KPFM has been implemented in differentvariants, but common to all of them is that they minimize theelectrostatic force by nullifying this electric force induced mechanicaloscillation amplitude.

Pioneered by Weaver et al. in 1991, AM-KPFM includes applying an AC biasbetween the AFM probe and the sample, usually albeit not necessarily, ator near the mechanical resonance frequency f of the AFM cantilever tocause it to oscillate under the AC electric force there between. A DCbias voltage, also applied between the probe and the sample, isregulated by a KPFM feedback algorithm so that the oscillation at fstops. At this point, the AC electric force at frequency f is nullified,and the DC voltage applied equals exactly the CPD. As revealed by thefollowing equation below,

$V_{DC} = \frac{\Delta\varphi}{e}$

when amplitude of the f term drops to 0. AM-KPFM is therefore anull-force technique.

$\begin{matrix}{F_{el} = {{\frac{\partial C}{\partial z}\left( {\left( {V_{DC} - \frac{\Delta\varphi}{e}} \right)^{2} + {\frac{1}{2}{V_{AC}}^{2}}} \right)} +}} & {{DC}\mspace{14mu} {Term}} \\{{\frac{\partial C}{\partial z}\left( {V_{DC} - \frac{\Delta\varphi}{e}} \right)V_{AC}{\sin \left( {2{\pi {ft}}} \right)}} +} & {f\mspace{14mu} {Term}} \\{\frac{1}{4}\frac{\partial C}{\partial z}{V_{AC}}^{2}{\cos \left( {4{\pi {ft}}} \right)}} & {2f\mspace{14mu} {Term}}\end{matrix}$

As shown in FIG. 12, an AM-KPFM 50 includes a) an AFM control block 52configured to operate the AFM in either intermittent contact mode(AM-AFM) or non-contact mode (FM-AFM), and b) a KPFM control block 54.AM-KPFM 50 includes a probe 56 having a lever 58 supporting a tip 60that is caused to interact with a sample 62 (note the chargedistribution shown on the sample indicating electrical properties to bemeasured). In either AM-AFM mode or FM-AFM mode, an AC voltage isapplied to, for example, the tapping piezo 64 by source 63 to cause theAFM probe 56 to oscillate at or near its resonance frequency f₁.Deflection of probe 106 during operation is measured by directing alaser beam from source 68 toward the backside of lever 58 and towarddetector 70. The deflection signal from detector 70 is transmitted tosignal processing block 72 of AFM control block 52 to determine anappropriate control signal to maintain probe-sample interaction at asetpoint. This feedback control signal (together with a scanning controlsignal provided in block 74) is transmitted to an actuator 66 (e.g., anX-Y-Z piezoelectric tube) to appropriately position the probe 56supported thereby.

AM-KPFM control block 54 includes a source 78 which delivers an AC biasat a second frequency f₂, is applied between the probe and the sample,giving rise to an alternating electric force between probe 56 and sample62, causing the probe to oscillate at this frequency as well. Inoperation, the detected oscillation signal from detector 70 istransmitted to a lock-in amplifier 76 of control block 54 for comparisonto the AC bias output by source 78. An AM-KPFM feedback block 80generates an appropriate control voltage signal which is added to the ACbias at block 82. Control block 54 continues to adjust the DC bias sothat probe oscillation at f₂ drops to 0. At this point V_(dc) equals thecontact potential difference (CPD) between sample 62 and probe 56.

(ii) The so-called frequency-modulation method (FM-KPFM) detects theresonance frequency shift Δf induced by the bias voltage applied betweentip and sample. FM-KPFM is sensitive to the electric force gradient,which is much more confined to the tip front end than electric force.Hence, for the FM method higher lateral resolution than for theforce-sensitive AM method is expected.

Frequency-modulation KPFM (FM-KPFM) was introduced in 1991 byNonnenmacher et al, and was perfected under ultrahigh vacuum in 1998 byKitamura et al. (“High-resolution Imaging of Contact PotentialDifference with Ultrahigh Vacuum Noncontact Atomic Force Microscope”,Kitamura et al., Appl. Phys. Lett. Vol. 72, No. 24, June 1998, pp.3154-3156; and U.S. Pat. No. 6,073,485). Typically, the cantilever ismechanically driven by a tapping piezo at or near the resonancefrequency of the cantilever f, and an AC bias is applied between theprobe and the sample with a frequency f_(m) usually much lower than thefundamental probe resonant frequency. The AC bias modulates the electricforce gradient between the probe and the sample, thus periodicallychanging the effective spring constant of the cantilever; this causesthe resonance frequency to shift periodically, that is, to modulate, atf_(m) and 2f_(m). A DC voltage is adjusted by the KPFM feedbackalgorithm so that the frequency modulation at f_(m) stops. At thispoint, the electric force gradient is nullified, and the DC voltageapplied measures the CPD,

${V{dc}} = \frac{\Delta\varphi}{e}$

as expressed in the following equation.

$\begin{matrix}{\frac{\partial F_{el}}{\partial z} = {{\frac{1}{2}\frac{\partial^{2}C}{\partial z^{2}}\left( {\left( {V_{DC} - \frac{\Delta\varphi}{e}} \right)^{2} + {\frac{1}{2}{V_{AC}}^{2}}} \right)} +}} & {{DC}\mspace{14mu} {Term}} \\{{\frac{\partial^{2}C}{\partial z^{2}}\left( {V_{DC} - \frac{\Delta\varphi}{e}} \right)V_{AC}{\sin \left( {2f_{m}t} \right)}} +} & {f_{m}\mspace{14mu} {Term}} \\{\frac{1}{4}\frac{\partial^{2}C}{\partial z^{2}}{V_{AC}}^{2}{\cos \left( {4\pi \; f_{m}t} \right)}} & {2f_{m}\mspace{14mu} {Term}}\end{matrix}$

FM-KPFM is a null-force-gradient technique. As shown in FIG. 13, anFM-KPFM 100 includes a) an AFM control block 102 configured to operatethe AFM in either intermittent contact mode (AM-AFM) or non-contact mode(FM-AFM), and b) a KPFM control block 104. AFM 102 includes a probe 106having a lever 108 supporting a tip 110 that is caused to interact witha sample 112 (note the charge distribution shown on the sampleindicating electrical properties to be measured). In either AM-AFM modeor FM-AFM mode, an AC voltage is applied to the tapping piezo 114 bysource 113 to cause the AFM cantilever to oscillate at or near itsresonance frequency f₁. Deflection of probe 106 during operation ismeasured by directing a laser beam from source 115 toward the backsideof lever 108 and toward detector 117. The deflection signal fromdetector 117 is transmitted to signal processing block 118 of AFMcontrol block 102 to determine an appropriate control signal to maintainprobe-sample interaction at a setpoint. This feedback control signal(together with a scanning control signal provided in block 120) istransmitted to an actuator 116 (e.g., an X-Y-Z piezoelectric tube) toappropriately position the probe supported thereby.

FM-KPFM control block 104 includes a source 128 that provides an AC biasat frequency f₂, usually a few kHz, applied between the probe and thesample, giving rise to an alternating electric force gradient betweenprobe 106 and sample 112. This force gradient will cause the proberesonant frequency to modulate, manifested as sidebands at f₁±nf₂ whichare used for KPFM feedback. The sideband frequencies are known (Block124) and input to lock-in amplifier 122 for comparison to the outputsignal of detector 117. Preferably, a KPFM feedback block 126 operatesto continuously adjust the DC bias (which is added to the AC bias atblock 130) so that the probe response at the side bands (f₁±f₂) drops to0. When doing so, the V_(dc) equals the contact potential difference(CPD) between sample 112 and probe 106 providing one of the electricalproperties of the sample at that location.

One of the challenges with KPFM, FM-KPFM mode in particular, is that alow spring constant and high cantilever Q is desirable for makingsensitive high resolution measurements. However, though one of the majoradvantages of SPM over other high-resolution microscopes (such as SEM)is operation in air, SPM limits possible Q values. Therefore, to achievesensitive KPFM measurement capability, it is known that using a probewith the lowest possible spring constant can offset limited Q values ofthe SPM (theoretical explanation is detailed in the probe design sectionof this invention). However, for practical reasons (e.g., to helpprevent the probe tip from sticking to the sample as it makesintermittent contact therewith), TappingMode requires use of probeshaving relatively high spring constants for reliable operation; and nottoo high a Q value to attain a bandwidth that allows a reasonably fastscan rate. KPFM sensitivity is thereby necessarily limited. Contact modepermits the use of levers having lower spring constants but is generallyknown to be one of the most destructive SPM techniques and is thereforelimited with respect to which the types of samples with which it can beused.

KPFM accuracy and resolution can also be limited by any one or more ofthe following: tip wear, tip contamination (need stable tip workfunction), metal degradation particularly over the apex, parasiticcapacitance (compromise lateral resolution), parasitic electrochemistry,unintentional charge dissipation from the sample, etc. Applicantsrealized a probe design that overcomes these drawbacks would bebeneficial.

In the end, notwithstanding the broad application and capabilities ofAFM and KPFM, limitations with each have remained. Sensitivity is theprimary limitation of KPFM. Nanometer scale sample features have manyinteresting material properties and AFM has been one of the major toolsto characterize them. However, while AFM is reliable in providingmultidimensional information with very high resolution and has gainedbroad acceptance as the tool of choice for many applications in imaging(e.g., topography), AFM has not been as successful with respect toquantitative mechanical property characterization.

Another fundamental limitation of current KPFM is that it is integratedwith Tapping Mode (or intermittent contact mode or AC mode). Stabilityof the tapping mode critically depends on the spring constant of thecantilever probe k, where the common value of k is about 40 N/m and canbe reduced to 5 N/m with marginal performance. The sensitivity factor ofthe KPFM detection is defined by Q/k, where Q is the mechanical qualityfactor of the cantilever. Given a typical “Q” of 200 on surface, normalKPFM usually have a sensitivity factor from 5-40. Using probes having amuch lower spring constant is therefore desirable; however, low springconstant probes are incompatible with the requirements of Tapping Modefeedback stability.

More particularly, conventional AFM has been known for its inability tosimultaneously acquire both high-resolution images and quantitativemechanical property information (e.g., elasticity, plasticity, andadhesion). Measuring mechanical properties with an AFM experimentalsetup is possible, but most known methods and systems rely on collectingforce curves corresponding to the local tip-sample interaction, anextremely slow process.

Additionally, current KPFM measurement systems are subject to largevariations in the surface potential value, due primarily to changes atthe probe apex during imaging. For example, the surface potential ofgold (Au) is around 800 mV. A first concern is drift during measurement.Drift can be substantial, often exceeding hundreds of mV, thereby makingaccurate measurements using AFM impractical. In addition, the measuredsurface potential varies when a probe is replaced or used for anextended period of time, and can also vary from system to system. Thesechanges are typically caused by uncertainty and variation of theconductive coating on the AFM probe; in particular, the crystalstructure of the apex of the tip is poorly defined and changes fromprobe to probe.

Generally, as shown in FIG. 14, an AFM probe 600 consists of two parts,a cantilever 602 which is sensitive to the forces between the tip andthe sample, and a tip 604. The tip 604 includes a body 606 having a base608 that connects to or is otherwise support by cantilever 602 and anapex 610 having a radius in nanometer range. Apex 610 is the part of theprobe that interacts with the sample, with the resolution of the AFMsubstantially defined by the radii of the apex. Most KPFM measurementapparatus and methods utilize either i) a coated probe (conducting),where the conducting material at apex 610 is poorly defined dueprimarily to inherent imperfections in the coating process, or ii) anetched metal wire, the mechanical properties of which are poorlycontrolled during fabrication, as understood in the art. In either case,the KPFM measurement is compromised.

In sum, the microscopy field has been left wanting a more comprehensiveinstrument capable of fast, high sensitivity electrical, topography, andmechanical sample property measurement. Ideally, the tool would becapable of associating the measured electrical properties with thecorresponding mechanical properties of the sample at each dataacquisition location.

SUMMARY OF THE INVENTION

The preferred embodiments are directed to high performance KPFM. Moreparticularly, the invention is directed to an AFM that combinesmechanical property measurement of a sample, e.g., on the nanoscale,with the capability to characterize electrical properties using a Kelvinprobe configuration. The preferred embodiments make it possible tomeasure mechanical properties and electrical properties of a sample atthe same time, with improved accuracy/resolution, at the nanometerscale, and associate that electrical property data with more credibletopography and mechanical property information concerning the sample.

By employing an innovative mode of AFM operation, peak force tappingmode (PFT mode), the exemplary embodiments are able to achieve highersensitivity factor as well as accuracy/repeatability improvements overknown systems, primarily due to PFT mode's ability to support probeshaving higher sensitivity factor. Moreover, the exemplary embodimentscombine the mechanical and electrical capabilities of AFM tosubstantially fully characterize samples on, e.g., the nanometer scale.While PFT methods and apparatus operate to gather data concerningmechanical properties of a sample and KPFM methods and apparatus operateto gather data concerning electrical properties of a sample, thepreferred embodiments make AFM more powerful by integrating the twotogether. Not only does the result improve user efficiency, but alsoallows in situ correlation of mechanical properties and electricalproperties for very small features (i.e., high resolution), thusproviding new information regarding a material's characteristics andperformance.

Preferably, the probe of the exemplary embodiments has a tip including abody made of a homogeneous material (conducting) from its base to itsapex. Homogeneous material means the same chemical composition, physicalproperties (including conductivity), crystalline orientation and surfacepotential. In one embodiment, the tip is made of a homogeneousconducting material, with an insulating layer between the cantilever andtip base. The insulating layer can be optional but the homogeneousmaterial of the tip (base to apex) must always be present. In analternative embodiment, the tip and the cantilever are made of onehomogeneous material that is conducting. Using either of theseembodiments of the probe, the accuracy of the present KPFM measurementis substantially improved.

In one aspect preferred embodiments, a method for measuring multipleproperties of a sample includes providing an atomic force microscope(AFM) including a probe having a cantilever and a tip. The method alsoincludes operating the AFM to cause the probe to interact with thesample in a two pass procedure. During a first pass of the two passprocedure, a surface of the sample is detected by operating the AFM inPFT Mode. Then, during a second pass of the two pass procedure,electrical property data corresponding to the sample is collected.

In accordance with another aspect of the preferred embodiments, themethod includes acquiring at least one of topography and mechanicalproperty data during the detecting step. The acquiring step may includecollecting mechanical property data and the mechanical property dataincludes at least one of elasticity, stiffness, plasticity,viscoelasticity and hardness.

In another aspect of the preferred embodiments, the probe has asensitivity factor (Q/k) greater than 40.

According to another aspect of the preferred embodiments, the secondpass of the collecting step includes using at least one of FM-KPFM andAM-KPFM. Notably, a DC bias employed in the second pass is set to zeroin the first pass, and an AC bias is preferably set to equal half thefundamental cantilever resonant frequency in the second pass.

In another aspect of the preferred embodiments, the probe includes acantilever and a tip, and the tip includes a body having a base and anapex, and wherein the body of the tip is made of a homogeneous material.

According to another aspect of this preferred embodiment, an insulatinglayer may be disposed between the cantilever and the tip. Alternatively,a combination of the cantilever and the tip may be made of a singlehomogeneous material, with no insulating layer.

A SPM configured in accordance with another preferred embodimentincludes providing an atomic force microscope (AFM) having a probedefining a tip. The material of the entire tip is homogeneous. Themethod also includes providing relative scanning motion between theprobe and a sample causing the probe to interact with the sample. TheAFM is operated to collect topography data, mechanical property data andelectrical property data with the probe in one of a group including asingle pass procedure and a two pass procedure.

In another aspect of the preferred embodiments, the operating stepincludes using PFT mode to collect the topography data and themechanical property data. More specifically, the operating step may beperformed as a two pass procedure using LiftMode™, and the topographydata collected in a first pass of the two pass procedure is used in thesecond pass.

According to another aspect of the preferred embodiments the second passincludes using FM-KPFM and wherein the FM modulation step includesproviding first and second lock-in amplifiers in a cascadeconfiguration.

In another aspect of the preferred embodiments, the probe has a springconstant less than 1 N/m.

According to a further aspect of the preferred embodiments, a method ofoperating a SPM includes a second pass of a two pass procedure using ahigh voltage detection circuit to measure a surface potential of thesample greater than ±12 volts.

According to a still further aspect of the preferred embodiments, thesecond pass includes applying an AC bias voltage between the probe andthe sample. In this case, the AC bias voltage has a frequency lower thanone-half the resonant frequency of the probe.

In a further aspect of the preferred embodiments, the method includesperforming a thermal tuning step to determine the fundamental resonantfrequency of the probe.

In another embodiment, a method for measuring multiple properties of asample includes providing an atomic force microscope (AFM) including aprobe having a tip, and operating the AFM to cause the probe to interactwith the sample in a one pass procedure. The method also includescollecting topographic and mechanical property data corresponding to thesample using PFT mode, and collecting electrical property datacorresponding to the sample with the probe using KPFM.

According to another aspect of the preferred embodiments, the probe hasan insulating cantilever with a conductive tip made of a single materialon one side, and a conductive coating on the other side made of a puremetal

In yet another embodiment, a method of operating an atomic forcemicroscope (AFM) to measure a sample, the method includes providing anAFM including a probe having a tip, wherein the entire tip is made of ahomogeneous material. The AFM is opened in peak force tapping (PFT)mode, and the method includes collecting KPFM data during said operatingstep.

According to another aspect of this embodiment, the method includesperforming a thermal tuning step to determine the fundamental resonantfrequency of the probe.

These and other features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription and the accompanying drawings. It should be understood,however, that the detailed description and specific examples, whileindicating preferred embodiments of the present invention, are given byway of illustration and not of limitation. Many changes andmodifications may be made within the scope of the present inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout, and in which:

FIG. 1 is a schematic illustration of a preferred embodiment of theinvention showing an AFM configured for KPFM operation using PFT mode tocollect topography, mechanical property and electrical property data;

FIG. 2 is a schematic illustration of an exemplary embodiment of the AFMof FIG. 1, configured as a PF-FM-KPFM;

FIG. 3 is a schematic illustration of an exemplary embodiment of the AFMof FIG. 1, configured as a PF-AM-KPFM;

FIG. 4 is a schematic illustration of an apparatus for providing FMmodulation in the exemplary embodiment shown in FIG. 2;

FIG. 5 is a flow chart illustrating a two-pass KPFM method according toan exemplary embodiment;

FIG. 6 is a flow chart illustrating a single-pass KPFM method accordingto an exemplary embodiment;

FIG. 7 is a schematic diagram of a preferred embodiment of a probeaccording to a preferred embodiment;

FIG. 8 is a schematic illustration of an alternative embodiment forperforming high voltage KPFM measurements;

FIG. 9 is a flow chart illustrating a two-pass KPFM method associatedwith the alternative embodiment shown in FIG. 8;

FIG. 10, is a flow chart illustrating a single pass KPFM methodassociated with the alternative embodiment shown in FIG. 8;

FIG. 11 is a schematic illustration of a prior art atomic forcemicroscope (AFM);

FIG. 12 is a schematic illustration of a prior art KPFM, namely, anAM-KPFM;

FIG. 13 is a schematic illustration of a prior art KPFM, namely, anFM-KPFM; and

FIG. 14 is a schematic illustration of a prior art AFM probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The benefits of PFT mode AFM are numerous. Most noteworthy is itscapability of simultaneous quantitative mechanical property mapping andtopographical imaging. Moreover, its ability to use cantilevers havingproperties (spring constant, resonant frequency and quality factor) overa wide range allows for probe selection most suited to KPFM operation.It is the object of this invention to combine PFT mode AFM with KPFM,preferably FM-KPFM, and AM-KPFM as an alternative. This will offersimultaneous surface topography, mechanical properties, and surfacepotential (electrical property) mapping with enhanced sensitivity.Ease-of-use operation also benefits beginner users for getting highquality data without intensive learning and practicing. Theimplementation and benefits will be outlined below.

Peak Force Tapping Mode (PFT mode), provides a solution to thequantitative mechanical property mapping. With the tip driven in and outof the contact with the surface at multi-kilohertz frequency, the tip'sposition and mechanical response (bending and thus reflection) arerecorded. The recorded data resembles conventional force curve data andare thus analyzed based on a well-defined model. Mechanical properties,such as elasticity, plasticity, adhesion, etc., can be derived for thelocalized area under the probe's apex. It is notable that the process tocapture data and do the analysis is done at very high speed(sub-milliseconds) and therefore quantitative mapping with high specialresolution is achieved.

AFM Peak Force Tapping (PFT) mode is described in U.S. Ser. No.12/618,641 filed Nov. 13, 2009, entitled Method and Apparatus ofOperating a Scanning Probe Microscope and U.S. Ser. No. 13/306,867,filed Nov. 29, 2011, and entitled Method and Apparatus of Using PeakForce Tapping Mode to Measure Physical Properties of a Sample. Using PFTMode, the AFM drives the cantilever at a frequency far lower than theresonant frequency, contrary to TappingMode, allowing essentiallyinstantaneous force monitoring and control. Sample imaging andmechanical property mapping are achieved with improved resolution andhigh sample throughput, with operation suitable in air, fluid and vacuumenvironments. Moreover, PFT mode facilitates ease-of-use operation,where an algorithm may be employed to automatically adjust the AFMimaging feedback gain, force setpoint and scan rate based on apredetermined noise threshold.

Though PFT mode provides substantial advantages regarding mechanicalproperty characterization on the nanometer scale, it is unable toprovide all data some users need. For instance, characterization ofelectrical properties of the sample may be desired, includingassociating one or more electrical properties with one or morecorresponding mechanical property characteristics (along withtopography) at each data collection point.

As discussed previously, Kelvin Probe Force Microscopy (KPFM) is anestablished method using AFM to measure some electrical properties suchas work function, electrical potential, local charge, dielectricconstant, and so on. It uses the same principle as the traditionalKelvin probe. The probe of the AFM serves as the reference electrodewhich forms a capacitor with the surface under test. Traditionalfeedback or LiftMode™ is used to keep the distance between the probe andthe sample surface constant. An alternating current (AC) voltage isapplied to the probe. When the potential between the probe and thesample surface are different, the applied AC voltage will cause thecantilever to vibrate. By detecting this vibration and providing anadditional DC offset to minimize it, the potential of the sample surfacecan be accurately measured. However, traditional KPFM does not giveinformation about a material's mechanical properties.

KPFM techniques using periodic excitations can benefit from operatingthe AFM so that the response of interest occurs at or near a cantileverresonance. At resonance, the cantilever's amplitude response is

${x = {Q\frac{F}{k}}},$

i.e., enhanced by a factor Q over the steady-state result derived fromHook's Law,

$x = {\frac{F}{k}.}$

Here, F and x are the amplitudes of the sinusoidal force on thecantilever and the resulting displacement, respectively. Thecantilever's spring constant is k and Q is the quality factor of itsassumed resonance. The best-case signal-to-noise ratio (S/N) atresonance of the detection of the displacement, x, can be estimated fromthe thermal noise using the equipartition theorem: k_(B)T/2=k(Δx)²/2,where k_(B) is the Boltzmann constant, T is the absolute temperature,and Ax is the expected noise amplitude. Therefore

$\frac{x}{\Delta \; x} = {\frac{Q}{\sqrt{k}}{\frac{F}{\sqrt{k_{B}T}}.}}$

These relations reveal that higher Q and lower k will benefit AM-KPFMsensitivity and detection limit (assuming at S/N=1).

The following analysis helps understand how the characteristics of thecantilever affect FM-KPFM sensitivity. For an AFM cantilever with aspring constant k and an effective mass m, its mechanical resonancefrequency is:

$f = {\frac{1}{2\pi}\sqrt{\frac{k}{m}}}$

An external long range force such as electrostatic force with gradient

$\frac{\partial F_{el}}{\partial z}$

gives rise to a frequency shift:

${\Delta \; f} = {{\frac{f}{2k}\Delta \; k} = {\frac{f}{2k}\frac{\partial F_{el}}{\partial z}}}$

The frequency shift corresponds to a phase shift, which is commonly usedin FM-KPFM detection. In a harmonic oscillator, a resonance frequencychange across its bandwidth ω/Q corresponds to a phase shift of 90°,therefore,

$\begin{matrix}{{\Delta\varphi} = {90\frac{\Delta\omega}{\omega/Q}}} \\{= {90Q\frac{\Delta\omega}{\omega}}} \\{= {90Q\frac{\Delta \; k}{2k}}} \\{= {45\frac{Q}{k}\frac{\partial F_{el}}{\partial z}}}\end{matrix}$

For a given electric force gradient, a bigger sensitivity factor Q/k ofthe cantilever leads to a bigger phase change, and thus highermeasurement sensitivity for FM-KPFM.

From these expressions it is clear that a high Q and low k is desirablefor sensitive AM-KPFM, and particularly FM-KPFM measurements.

FM-KPFM under vacuum (absence of air damping) enjoys high sensitivitythanks to the high Q (usually 2-3 orders of magnitude higher than inair). However, for SPM operation in air, which is one of the majoradvantages of SPM over other high-resolution microscopes such as SEM,potential Q values are limited. Hence, the lowest possible springconstant is important for sensitive KPFM in air. For practical reasons,standard TappingMode SPM operation requires using probes havingrelatively high spring constants for reliable operation (e.g., the probetip may stick to the surface of the sample), and not too high a Q valueto attain a bandwidth that allows a reasonably fast scan rate.Therefore, KPFM sensitivity is necessarily limited.

PFT mode AFM lifts the restrictions associated with intermittent-contactmode (TappingMode). As a result, probes having a wide range ofcharacteristics can be used to enhance KPFM detection sensitivity. Forinstance, typical KPFM probes have a sensitivity factor Q/k of around40. Now, probes having a corresponding Q/k ratio above 40, as well asabove 100 and even 200, can be employed with the present preferredembodiments.

The construction of the probe impacts performance as well. Inconventional KPFM, the probe tip is not homogeneous; rather, it may be,for example, a silicon tip with a metal coating to make the tipconductive. It has been discovered that performing KPFM using a probehaving an inhomogeneous tip can severely limit the accuracy of theacquired KPFM data. In the present preferred embodiments, the probe ismade to be homogeneous. “Homogeneous” in the context of the presentapplication means that the probe is made of a single material with thesame crystalline structure throughout its volume. In this case, the tipwhich includes a body defining a base (the base being coupled to thecantilever, either directly, or indirectly with an intermediateinsulating layer (described further immediately below)), and an apex(the distal end of the probe tip which interacts with the sample duringPF-KPFM operation).

An illustration of a probe design that overcomes some of the drawbacksof conventional AFM probes used in KPFM is shown in FIG. 7. Unlike priorprobes with metal coatings (platinum or gold—possible work functiondifference between the coating and the probe silicon, especially ifcoating is scratched), a probe 400 such as that shown in FIG. 7 as partof a KPFM configuration with corresponding V_(dc) and V_(ac) biassources 412, 414, respectively, is a capacitive probe that providesseveral advantages. Probe 400 includes a cantilever 402 made of, forexample, a silicon nitride insulating lever 406 with a tip made of asingle homogeneous material (doped silicon, or pure metal) 404 extendingtherefrom. A pure metal (e.g., aluminum) can be deposited on thebackside or top surface of lever 402 to form a capacitor with tip 404.

With insulating layer 406, probe 400 operates to minimize current flowbetween the tip and sample. As a result, the chance that anelectrochemical reaction occurs at the sample surface is minimized, andthus stability is improved. Moreover, when using probe 400, shifts inmeasured potential due to tip wear are lessened given that the workfunction of the probe is dictated by the back side coating material.When employing probe 400, KPFM with 20 mV accuracy/repeatability, orbetter, can be achieved. This probe will also limit charge dissipationof the sample.

Notably, probe 400 does not require insulating layer 406 (and thus isshown as optional in FIG. 7). In an alternative without layer 406, theentire probe 400 is made of a single homogeneous material. For example,the entire probe could be made of a metal or it could be made of anappropriately doped silicon, with no insulating layer, with a metal(such as aluminum) deposited on its backside for optical detection ofprobe deflection. In this case, the tip remains homogeneous and thus thework function of the probe material remains constant and accurate KPFMdata (e.g., to 50 mV) can be obtained.

In yet another alternative, a capacitor 410 may be added in series to aregular conductive probe to achieve similar effect, though care must betaken with capacitor selection or compromised data (e.g., streaks) mayresult due to static charge hang-over in capacitor 410.

Referring to FIG. 1, a combination of peak force tapping technology andKelvin probe measurement is shown as a KPFM instrument 150 including acontrol block 160 and a data collection unit 162. In one embodiment, theintegration of these two state-of-the-art techniques is realized throughLiftMode™ operation, i.e., a two pass procedure. Note that herein theterms “single pass” procedure and “two (or dual) pass” procedure areused. These terms refer to the relative scanning motion between theprobe and the sample (in XY) during AFM operation being performed eitheronce or twice on the same scan line in a raster scan (e.g., LiftMode™).

In FIG. 1, during a first pass, PFT operating mode is employed todetect/determine the sample surface which often includes acquiringaccurate surface topographical information, as well as mechanicalproperties (via force curves at each X-Y location). More particularly,KPFM 150 includes a probe 152 defining a cantilever 154 supporting a tip156 at its distal end. Probe 152 is scanned across the surface of asample 158 while the probe is oscillated generally at a multi-kilohertzoff-resonance frequency. The deflection of the lever 154 is monitoredand sent to a PFT mode control block 164 which operates to keep the tipsample force at the PFT setpoint. As understood, it is the controlprovided by PFT mode that may yield signals indicative of mechanicalproperties of the sample surface, as well as topography. This data isstored in data collection unit 162 depicted in FIG. 1 as blocks 166 and168, respectively.

Then, once the surface topography is known at each data collection point(X, Y) along one scan line on the sample surface, the tip preferably islifted up some constant distance “Z” from the surface to follow thesurface profile on a second pass/scan of sample surface 158 during whicha Kelvin probe measurement is made using a KPFM algorithm 170. Note thatif the sample surface is merely determined in the first pass, a simplelift at a user-selected distance may be employed in the second pass ofthe scan, i.e., topography data, though preferred, need not be collectedand used in the second pass; rather, the lift and second pass may beperformed regardless of whether the topography is known from the firstpass. The electrical property, mechanical property and topographyinformation can then be combined to render a composite view of differentsurface features of sample 158. Implementations of the KPFM, including aPF-FM-KPFM (FIG. 2) and a PF-AM-KPFM (FIG. 3) are described below.

Referring initially to FIG. 2, a PF-FM-KPFM 180 includes PFT mode AFMhardware including a probe 182 defining a cantilever 184 supporting atip 186. In this two-pass embodiment of the present invention, an ACbias is applied to an actuator 192 coupled to probe 182 to oscillateprobe 182 at a multi-kilohertz frequency during a first pass (governedby control block 198; control block 206 is the KPFM control block). Tip186 of PF-FM-KPFM 180 is thereby caused to interact with the surface ofa sample 188. As probe tip 186 interacts with the surface of sample 188,the deflection of probe 182 is monitored by directing a laser beam froma source 194 toward the backside of lever 184, which is then reflectedto a detector 196, such as a quadrant photo detector 196. Detector 196transmits this deflection signal to a peak force algorithm block 200.Peak force algorithm block 200 generates a signal indicative of theforce corresponding to the detected deflection, and that force signal iscompared to a force setpoint at block 202. A PFT mode controller 204then determines an appropriate control signal “S” based on the detectedforce which is transmitted to an actuator 192 (e.g., a piezoelectric XYZtube) to appropriately position probe 182 in “Z.” At each X-Y locationof the sample, the interaction is captured to generate a force curvefrom which several mechanical properties of the sample can be derived.Note that KPFM block 206 is not operational during the first pass inwhich the DC bias (between the probe and the sample is maintained (e.g.,set) at zero.

PFT mode can be performed automatically, in which at least one of thefeedback gain, scan rate, and peak force setpoint can be set by thesystem software. Moreover, though preferred, mechanical property mappingneed not be included. When it is, PF-KPFM provides simultaneous (withtopography imaging) property mapping of at least one of adhesion,elasticity, hardness, plasticity, surface deformation and energydissipation, for example.

During a second pass over the sample scan line, probe 182 is “lifted” afixed distance “z” (usually a few nanometers, up to a few hundrednanometers) from the surface. An AC signal at frequency f₁ is applied tothe tapping piezoelectric actuator 190 which oscillates the probe at ornear its mechanical resonance frequency f₁. A second AC bias signal atfrequency f₂ is applied to the sample which produces an AC electricfield between the probe and the sample. The overall effect is a proberesponse with side bands at f₁±nf₂ frequencies. The KPFM feedback schemecontinues to adjust the DC bias so that the side bands at f₁±f₂ vanishesmanifesting that the electric force gradient is nullified. The potentialat the surface of sample 188 at that XY location is thereforequantified/measured, i.e., the applied DC voltage equals the CPD.

Alternatively, a dual frequency AC bias can be used: with the firstfrequency at half the resonance frequency of the cantilever, whichreplaces the mechanical drive to cause the probe to vibrate at itsresonant frequency; and the second frequency again at a few kilohertz.

In another embodiment of the invention, a PF-AM-KPFM 220 is employed asshown in FIG. 3. PF-AM-KPFM 220 includes PFT mode AFM hardware includinga probe 222 defining a cantilever 224 supporting a tip 226. As tip 226interacts with a surface of a sample 228, the deflection is monitored,for example, by providing a laser source 234 which directs light towardthe backside of cantilever 224 for reflection to a detector 236.

In this case, during the first pass, the Z-position of probe 222 iscontrolled by PFT feedback to follow the sample surface. Probe 222 ismade to oscillate in the “Z” direction to periodically touch the surfaceof sample 228. PFT feedback is implemented using PFT algorithm at block240 which generates a force signal in response to the deflection signal,the force signal being compared to a force setpoint at block 242. Basedon the output of comparison circuit 242, a controller 244 determines anappropriate PFT control signal “S” to be applied to an actuator 232 (XYZpiezoelectric tube, for example) to adjust the Z-position of probe 222coupled thereto to maintain the tip-sample force at the setpoint. Ateach X-Y location, the interaction may be captured to generate a forcecurve from which mechanical properties can be derived. Note that KPFMblock 246 is not operational during the first pass in which the DC bias(between the probe and the sample is maintained (e.g., set) at zero.During a second pass, the cantilever is lifted a fixed distance “z” fromthe sample 228. An AC bias signal at frequency f₁ from source 250 isapplied between the probe and the sample at frequency f₁. A KPFMfeedback algorithm 248 (implemented in digital or analog circuitry)determines a DC bias based on the detected deflection of probe 222, theDC bias being combined with the AC bias at block 252 to continuouslyadjust the DC bias so that the probe's oscillation at f₁ is minimized.When f₁ is minimized, there is no DC electrical field between the probeand the sample. In this case, the potential at the sample surface atthat XY location is therefore quantified/measured, i.e., the applied DCvoltage equals the CPD.

Alternatively, KPFM including both PF-FM-KPFM and PF-AM-KPFM can operatewith feedback off thereby essentially reducing KPFM to an electric forcemicroscope (EFM), where phase or amplitude will be measured instead ofpotential.

Turning next to FIG. 4, the FM modulation/demodulation in the PF-FM-KPFMembodiment shown in FIG. 2 is shown. Two lock-in amplifiers 278, 280 incascade are used to implement the FM demodulation (block 208, FIG. 2),and related feedback. AC signal 1 (f₁) is generated by source 276 andapplied to a piezoelectric actuator 268 (tapping piezo) supporting aprobe 262 defining a lever 264 having a tip 266 at its distal end.Deflection of the probe is monitored by an optical detection schemeincluding a laser 272 which directs a light beam at the backside oflever 264 so it is reflected toward a detector 274. An actuator 268(e.g., a tapping piezoelectric actuator) oscillates probe 262 at theprobe's resonant frequency. At substantially the same time, a second ACsignal at frequency f₂ generated by a source 282 is applied to a sample270. This causes the frequency at which probe 262 oscillates to vary,which is reflected in the phase change of the f₁ component and can bedetected by Lock-in Amplifier (LIA) one 278. The output of LIA 278 canbe phase, amplitude, in-phase and quadrature, but preferably phase.

The output of LIA 278 is then fed to Lock-in amplifier two 280 todetermine its amplitude (A_(phase)) at the f₂ frequency, which isessentially the amplitude at sidebands f₁±f₂. This is used by thefeedback algorithm of controller 284 to determine an appropriate DC biasto be applied to sample 270 (combined with AC Signal 2 at block 286) tonullify A_(phase). As a result, the potential of sample 270 isquantified/measured with reference to probe 262. Note that while twoLIAs in cascade are shown and described, alternatives are contemplated.For instance, a combination of a filter and a lock-in amplifier could beused, as well as a combination of a frequency-voltage converter and alock-in amplifier.

A method 290 of operating the KPFM according to a preferred embodimentis shown in FIG. 5. In this two pass approach/procedure, after aninitialization and start up step at Block 292, the laser for the opticaldetection set-up is aligned with the probe and the method auto adjuststhe PF-KPFM (FM or AM) operating parameters in Block 293. A fast thermaltune algorithm is also employed to determine the thermal peak frequency(fundamental resonant frequency of the probe) which is used as the ACbias drive frequency (f₁) by either a) drive 214 of PF-FM-KPFM of FIG.2, or b) drive 250 of PF-AM-KPFM of FIG. 3. The fast thermal tunealgorithm is described in U.S. Prov. Pat. Appl. 61/558,970, filed onNov. 11, 2011, which is hereby expressly incorporated by reference, andis operated to collect relatively small chunks of data (e.g., 500 ms asopposed to the several seconds, or tens of seconds of data typicallycollected.) The KPFM AC bias drive amplitude (drive 214 of FIG. 2) isalso auto adjusted, while the AC bias phase offset is auto adjusted(drive 214 of FIG. 2, 250 of FIG. 3) as well, both in Block 293. Lastly,if a probe needs to be swapped out, this also is performed as part ofBlock 293 of method 290.

Notably, by providing robust data collection components, the preferredembodiments are capable of collecting absolute value KPFM electricaldata with 20 mV accuracy. Advantageously, as a result of thisaccuracy/repeatability improvement over known systems, the KPFMapparatus and methods of the preferred embodiments are completelyinstrument and probe independent, facilitating significant improvementsin operator ease of use.

Method 290 then operates to engage surface at Block 294, for example,using a rapid engage algorithm such as that shown and described in U.S.Pat. No. 7,665,349. Relative scanning motion between the probe andsample is initiated and AFM is operated in PFT mode as part of a firstpass in Block 298. As part of this 1^(st) pass in Block 298, the biasvoltage is set to zero. Once the sample surface data is acquired, thetopography is known and the probe is lifted off the surface a selecteddistance in Block 300. As stated previously, the distance the probe islifted off the surface can be user-selected independent of the sampletopography; for example, in the case in which topography data is notacquired and the surface is simply sensed in the first pass. Then, theKPFM is operated as part of a second pass of relative motion between thesample and probe in Block 302. As part of this 2^(nd) pass, the biasvoltage is applied. KPFM data can then be collected and stored accordingto the above described techniques in Block 304.

FIG. 6 illustrates a preferred embodiment in which a single pass isemployed to collect topography, mechanical and electrical property dataconcerning the sample surface. More particularly, a method 310 includesa start up and initialization step at block 312. Thereafter, the probeand sample are engaged with one another in block 314. Next, in block316, both the KPFM algorithm and the PFT algorithm are operatedsubstantially simultaneously to acquire topography, mechanical propertyand electrical property data in a single pass. The data acquired inblock 316 is then collected and stored for each XY position in block318. Generally, the method 290 illustrated in FIG. 5 may in some casesbe preferred to minimize adverse effects due to crosstalk.

High-Voltage KPFM

Known KPFM technology is capable of making voltage measurements up to a±12V. With the present embodiment, making voltage measurements up totens of volts (e.g., surface charge on a polymer), and even hundreds ofvolts, is possible. A high voltage KPFM instrument (KPFM-HV) 500 isshown in FIG. 8 and described below. KPFM-HV 500 is preferablyconfigured to operate as a two-pass method employing PFT mode AFM;however, as an alternative, AM-AFM or FM-AFM may be used. The first passuses feedback control as part of an AFM configuration 501 to determinephysical properties of the sample, including topography, while thesecond pass employs a high voltage detection circuit 502 to collect KPFMdata.

More particularly, in the first pass of probe-sample interaction, if PFTmode is employed, at least one of surface topography and well definedmechanical property information (adhesion, etc.) is obtained. If eitherAM-AFM mode or FM-AFM mode is employed, surface topography withunidentifiable mechanical properties are obtained. A probe 504 includinga cantilever 505 supporting a tip 506 is caused to interact with asample 508 by driving it in to oscillation using a tapping piezo 516 orother piezoelectric actuator. Deflection of probe 504 is detected bydetector 512 and transmitted to a signal processing block 518 thatoutputs a force control signal (PFT mode) that together with the scancontrol signal from position control block 520 appropriately positionsthe probe relative to the sample via appropriate signals sent to XYZactuator 514. In the second pass, an AC bias at a frequency lower thanhalf of the cantilever resonant frequency is applied between the probeand the sample via source 527. This AC bias causes the relative motionbetween the probe and sample to oscillate at a frequency f, as well asat its 2nd harmonic. The amplitude at these frequencies is determinedusing a pair of lock-in amplifiers, Lock-In Amplifier one 522 (AC biasfrequency (FM) 1-20 kHz, phase: +90°) and Lock-In Amplifier two 524,respectively. The electric potential between the tip and sample can becalculated at Block 526 based on the oscillation amplitude atfrequencies f and 2 f In particular, potential

${Potential} = {{{{sign}\left( {{Phase}\; 1} \right)} \star {\frac{1}{4}V_{ac}\frac{A_{f}}{A_{2f}}}}_{V_{{dc} = 0}}}$

and the voltage/potential data is stored at block 528. Notably, whetherPFT feedback is used or not, KPFM feedback is not employed in this highvoltage detection regime.

FIG. 9 is directed to a method 530 associated with the high voltageKPFM-HV shown in FIG. 8, using a two-pass approach (LiftMode™). After astart-up and initialization step 532, method 530 engages the probe onthe sample surface in Block 534. Relative scanning motion between theprobe and the sample is provided in Block 536 (raster scan, e.g.) whilePFT mode feedback is provided in Block 538. During this first pass,surface topography and mechanical property data is collected. In Block540, the probe is lifted a certain amount “z” and a second pass isinitiated. During this second pass, the amplitude response of the probeis determined at frequencies f and 2 f using lock-in amplifiers (FIG.8). Method 530 then calculates potential in Block 542. The data is thencompiled to generate a 3D topography map and a 2D mechanical propertymap from the PFT mode control signals, and a corresponding 2D potentialmap from the 2^(nd) pass data, in Block 544.

FIG. 10 is directed to a method 550 associated with the KPFM-HV shown inFIG. 8, using a single pass approach. After a start-up andinitialization step in Block 552, method 550 provides relative scanningmotion between the probe and the sample in Block 554. In this case, bothPFT mode feedback and KPFM-HV detection circuitry are operatedsimultaneously in Blocks 558 and 562, respectively. The potential iscalculated in Block 564 and the data is then compiled in Block 560 togenerate a 3D topography map and a 2D mechanical property map from thePFT mode control signals, and a corresponding 2D potential map from theKPFM-HV detection branch.

Advantages

The preferred embodiments offer simultaneous acquisition of surfacetopography, mechanical properties, and surface potential (electricalproperty) mapping. PFT mode AFM's ability to use cantilevers havingproperties (spring constant, resonant frequency and quality factor) overa wide range can be used to the advantage of KPFM measurement. Forinstance, probes with low spring constant and high quality factors thatare restricted for Tapping mode operation, now can be used to enhanceKPFM detection sensitivity.

The preferred embodiments also improve the KPFM measurementrepeatability by extending the lifetime of the probes. As discussedabove, the force exerted to the tip and sample can be much smaller inpeak force tapping mode than in tapping mode or contact mode. Tip wearand tear is therefore greatly reduced, which benefits KPFM spatialresolution (tip remains sharp over long scan time) and measurementconsistency.

Ease-of-use is another advantage. Traditional KPFM uses TappingMode orcontact mode to acquire the surface profile data. Tapping Mode iscomplicated by (a) indirect force control, (b) cantilever resonancedynamics of multiple harmonics, and (c) amplitude or phase of the probeoscillation during data acquisition can be affected by many factorsother than the tip-sample interaction force. Due to these complications,subjective judgment must be employed, most often requiring muchknowledge and experience. Even in contact mode AFM, constant drift ofthe cantilever deflection due to thermal or other system factors makesaccurate force control generally impossible. With the present method,PFT mode is used to acquire topographic data. PFT mode eliminates manyof the complications discussed above. The criteria used to judge optimalimaging parameters become simple and objective. As a result, themeasurement procedure can be automated.

In addition, the resonant frequency of the probe may be readilydetermined using thermal tuning (automatically). With respect to thelock-in amplifier configuration, the phase can be set automatically.While the mechanical drive amplitude is preferably automatically set todrive probe oscillation at optimal amplitude to enhance operationalconsistency.

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the above invention isnot limited thereto. It will be manifest that various additions,modifications and rearrangements of the features of the presentinvention may be made without deviating from the spirit and the scope ofthe underlying inventive concept.

What is claimed is:
 1. A method of operating a scanning probemicroscope, the method comprising: providing an atomic force microscope(AFM) including a probe having a tip, wherein the material of the entiretip is homogeneous; providing relative scanning motion between the probeand a sample causing the probe to interact with the sample; andoperating the AFM to collect topography data, mechanical property dataand electrical property data with the probe in one of a group includinga single pass procedure and a two pass procedure.
 2. The method of claim1, wherein the operating step includes using PFT mode to collect thetopography data and the mechanical property data.
 3. The method of claim2, wherein the operating step is performed as a two pass procedure usingLiftMode™, and the topography data collected in a first pass of the twopass procedure is used in the second pass.
 4. The method of claim 3,wherein the second pass includes using FM-KPFM and wherein the FMmodulation step includes providing first and second lock-in amplifiersin a cascade configuration.
 5. The method of claim 1, wherein the probehas a spring constant less than 1 N/m.
 6. The method of claim 5, whereinthe operating step is a two pass procedure including a first pass and asecond pass, and wherein the second pass includes using a high voltagedetection circuit to measure a surface potential of the sample greaterthan ±12 volts.
 7. The method of claim 5, wherein the operating step isa two pass procedure including a first pass and a second pass, andwherein the second pass includes applying an AC bias voltage between theprobe and the sample, the AC bias voltage having a frequency lower thanone-half the resonant frequency of the probe.
 8. A method for measuringmultiple properties of a sample, the method comprising: providing anatomic force microscope (AFM) including a probe having a tip; operatingthe AFM to cause the probe to interact with the sample in a one passprocedure; collecting topographic and mechanical property datacorresponding to the sample using PFT mode; and collecting electricalproperty data corresponding to the sample with the probe using KPFM. 9.The method of claim 8, wherein the probe has an insulating cantileverwith a conductive tip made of a single material on one side, and aconductive coating on the other side made of a pure metal.
 10. Themethod of claim 8, wherein KPFM is one of amplitude-modulation KPFM andfrequency-modulation KPFM.
 11. The method of claim 8, wherein theoperating step includes using PFT mode to collect the topography dataand the mechanical property data.
 12. The method of claim 11, whereinthe operating step is performed as a two pass procedure using LiftMode™,and the topography data collected in a first pass of the two passprocedure is used in the second pass.
 13. The method of claim 12,wherein the second pass includes using FM-KPFM and wherein the FMmodulation step includes providing first and second lock-in amplifiersin a cascade configuration.
 14. The method of claim 8, furthercomprising performing a thermal tuning step to determine the fundamentalresonant frequency of the probe.
 15. A method of operating an atomicforce microscope (AFM) to measure a sample, the method comprising:providing an AFM including a probe having a tip, wherein the entire tipis made of a homogeneous material; operating the AFM in peak forcetapping (PFT) mode; and collecting KPFM data during said operating step.16. The method of claim 15, further comprising performing a thermaltuning step to determine the fundamental resonant frequency of theprobe.
 17. The method of claim 15, wherein the operating step isperformed as a two pass procedure using LiftMode™, and the topographydata collected in a first pass of the two pass procedure is used in thesecond pass.
 18. The method of claim 17, wherein the second passincludes using FM-KPFM and wherein the FM modulation step includesproviding first and second lock-in amplifiers in a cascadeconfiguration.